1. Field of the Invention
The present invention relates to a developing device that supplies developer to a developer support member such as a developing sleeve while transporting the developer by a transport screw, and to an image forming apparatus using this developing device.
2. Description of the Related Art
Well-known in the past were developing devices using developer containing toner and magnetic carrier that, as developer is transported in the axial linear direction of rotation based on a supply transport screw arranged on the side of the developer support member, developer is supplied to the full longitudinal region of the developer support member. In this type of developing device, while the supply transport screw supplies developer to the developer support member, developer after participating in development is recovered from the developer support member. Then, developer transported to the downstream terminal in the supply direction of the supply transport screw is transferred by a separate transport screw. Next, after being transported while mixing with supplemental toner, the transferred developer is returned to the upstream terminal in the supply direction of the supply transport screw.
By conducting this kind of developer recirculation transport, as developer with the toner concentration depleted by development is recovered from the developer support member, developer that has had the toner concentration restored with supplemental toner can be supplied to the developer support member.
Nonetheless, in the related configuration the developer concentration is depleted little by little from the upstream to the down stream developer transport direction of the supply transport screw. Most of the developer in the system is the developer immediately after concentration restoration, and the percentage of used developer recovered from the developer support member goes on increasing in the vicinity the upstream terminal in the developer transport direction of the supply transport screw, as it turns to faces downstream in the developer transport direction.
Meanwhile, developing devices (called one-directional circulation systems) that recover developer from the developer support member and supply developer to the developer support member by separate transport screws are also well-known. One-directional circulation systems can suppress development concentration irregularities in the developer transport direction because used developer from the developer support member is not returned to the supply transport screw and the toner concentration of the developer on the supply transport screw can by stabilized in the developer transport direction.
For example, described in Japanese Laid-open Patent Publication No. 2001-249545 is a configuration that realizes a one-directional recirculation system having a supply transport screw that supplies developer to a developer support member, a receiving transport screw arranged directly below, and an inclined transport screw arranged at the side of these vertically aligned screws and oriented at an incline in relation to horizontal. In this configuration, the receiving transport screw restores the toner concentration by supplementing the used developer received from the developer support member with toner, and then transfers the developer to the inclined transport screw. The inclined transport screw transports the developer sent from the receiving transport screw at a rising grade to the supply transport screw arranged directly above the receiving screw.
In addition, in conventional image forming apparatuses such as copiers and printers, technology that suitably supplies new carrier to the developing device that houses 2 component developer comprising toner and carrier (called a “premix developing system”) is well-known, for example, Japanese Laid-open Patent Publication No. 2001-183893.
In the normal developing process, the carrier in two component developer housed in the developing device remains in the developing device without being consumed, and therefore the carrier deteriorates over time. Carrier deterioration modes include: film loss, in which the coating layer of the carrier is abraded or peels off and the charge capacity of the carrier drops, and spent, in which components and additives of the toner adhere to the surface of the carrier and the charge capacity of the carrier decreases. The premix developing system is for the purpose of preventing a reduction of image quality of the output image caused by this kind of carrier deterioration over time, and by suitably supplementing new carrier (or new two component developer) into the developing device and suitably discharging part of the two component developer inside the developing device to outside of the developing device, this system reduces the amount of deteriorated carrier inside the developing device and maintains the amount and charge capacity of carrier housed inside the developing device.
Japanese Laid-open Patent Publication No. 2006-063645 is an example disclosing an apparatus that uses a premix developing system in the one-directional circulating system of the developing device disclosed in the previously described Japanese Laid-open Patent Publication No. 2001-249545. In this type of developing device, a developer discharge port is provided on the downstream side of the supply transport screw, and when the surface of the developer transported to that location exceeds a specified height, that developer (the excess amount of developer due to the supplementation of carrier) overflows and is discharged to outside of the developing device.
In the configuration described in the previously described Japanese Laid-open Patent Publication No. 2006-063645, which uses a premix developing system in a developing device with a one-directional recirculating system, a developer discharge port is provided on the casing side surface part equivalent to the downstream side of the supply transport screw, but in a developing device with a one-directional recirculating system the used developer from the developer support member is not returned to the supply transport screw, and therefore the amount of developer of the supply transport screw part on the downstream side of the supply transport screw is reduced and the surface of the developer becomes low.
Because that position is one in which it is difficult for the height of the surface of the developer to reflect the amount of developer in the developing device and also because in this configuration the developer at that position is discharged from the aforementioned discharge port, and no matter the amount of developer in the developing device, there is the problem that a phenomenon is produced in which the developer that is kicked up by the screw blade is directly discharged from the discharge port, and the amount of developer in the developing device is prone to fluctuate. If the amount of developer in the developing device is not stable, the amount of developer supplied to the developer support member becomes unstable, and deteriorated images are produced in which auger pitch irregularities are generated on the output image.